An oscillator generates a periodic signal. Accordingly, an oscillator must have a self-sustaining mechanism that allows its own noise to grow and eventually become a periodic signal. Oscillators having a periodic signal whose frequency falls in the radio frequency range (RF) are often referred to as RF oscillators.
Many RF oscillators use feedback circuits to generate the periodic signal. In these RF oscillators, a frequency-selective network such as an inductor-capacitor (LC) tank is included in the feedback loop in order to stabilize the frequency. The frequency-selective network is also called a "resonator." The nominal frequency of oscillation is often determined by the characteristics of the circuit including, for example, the resonance frequency of the LC tank.
Most discrete RF oscillators incorporate only one active device (e.g., a transistor). There are two reasons for using a one-transistor topology: noise is minimized and costs are reduced.
FIG. 1 illustrates common collector configuration of a traditional Colpitts oscillator that is well known in the art. This Colpitts oscillator has only one transistor, a bipolar junction transistor 12. The transistor 12 has its collector connected to a voltage source V.sub.cc. The base of the transistor 12 is connected to an inductor 14 via node 16. The inductor 14 has an internal resistance, shown representatively by resistor 18. Resistor 18 is not a resistance separate from the internal resistance of the inductor 14. A resistor 20 is connected between the voltage source V.sub.cc and the node 16. Node 16 is also connected to one side of a capacitor 22 and the other side of the capacitor is connected to the emitter of the transistor 12 through nodes 24 and 26. A capacitor 28 is connected between node 24 and ground. A resistor 30 is connected between node 26 and ground.
Such standard Colpitts oscillators are well known and their characteristics have been well studied. Colpitts oscillators behave in a predictable fashion and are easy to implement. Nodes 24 and 26 are physically the same node and carry the output voltage.
One advantage of this Colpitts oscillator is that it has a low output impedance and therefore is less influenced by the circuits which follow it. However, the output signal on node 26 is a single signal. Hence, if the circuit designer requires differential signals, this Colpitts oscillator cannot output such signals. Therefore, there is a need to have an oscillator circuit that can output differential voltage signals which are accurate and have good harmonic content. A signal having good harmonic content is one that has a primary resonant frequency and whose higher order harmonic frequencies are suppressed. It is desirable to have a "balanced" signal, that is, one whose two components are 180 degrees out of phase. Differential voltage signals that are not precisely out of phase result in reduced signal amplitude or phase errors which may degrade the quality of systems that use oscillators. For example, telecommunication and cellular telephone systems that use noisy or inaccurate oscillators may suffer from perceptibly degraded voice qualities.
The phase noise of an oscillator based on a LC tank usually depends on the Q of the tank. The higher the Q of the LC tank, the sharper the resonance and the lower the phase noise skirts. The Q represents how much energy is lost as the energy is transferred from the capacitor to the inductor and vice versa. The Q, phase noise and other attributes of Colpitts oscillators have been well studied and are well known to those of skill in the art.
Oscillators may be used to form other devices including voltage controlled oscillators (VCOs). On a larger scale, oscillators may be used in wireless communication systems such as mobile radio communication systems and cellular telephone systems. Hence, improvements in oscillators lead to improvements in other systems.